Process for stripping adhesive from a metal base



NOV- 1, 1966 H. J. PINE ETAL 3,282,765

PROCESS FOR STRIPPING ADHESIVE FROM A METAL BASE Filed NOV. l5, 1962 United States Patent O Filed Nov. 15, 1962, Ser. No. 237,995 2 Claims. (Cl. 156-344) This invention relates to a new and useful process for the recovery of a metal plate used for the support of polymerizable strata.' More particularly, it relates to a new and useful process for the recovery of the metal plate from a photopolymerizable printing plate and relief printing plate made therefrom.

In the manufacture of photopolymerizable printing plates of the type described in Smith, U.S. Patent 3,012,952 and Martin et al., U.S. Patent 2,927,022, the metal support may represent a substantial part of the total cost of the element. Therefore, it is of considerable economic importance to recover the metal support from elements which have either served their useful life as printing plates or which have been found to be defective at some stage of their manufacture or use. None of the methods of recovery known in the art has proven entirely satisfactory, particularly in avoiding damage to the metal or in removing the adhering strata in a single sheet. Breaking -up of the adhering strata into sma'll pieces is especially objectionable in recovery systems using solvents since it causes rapid deterioration of the solvent.

An object of the present invention is to provide a practical and dependable process for recovering a metal support from a composite element comprising a stratum adhered to said support. Another object is to remove the stratum as a single sheet rather than in small pieces. Still another object is to provide a simple and rapid process for recovery -of the metal support in an undamaged form. Other objects and advantages of the invention will be apparent from the following description when read in connection with the accompanying drawing in which:

FIGURE 1 shows in enlarged cross-section the photopolymerizable and adhesive stratum on a metal support with a portion partially removed.

FIGURE 2 shows in enlarged cross-section a printing relief stratum and an adhesive stratum and the metal support.

These and other objects are accomplished by the novel process of this invention. This invention comprises a process for the recovery of a metal supporting sheet from an adherent polymerizable stratum which comprises polymerizing said strat-um and then separating said stratum in one piece from said metal supporting plate.

In a preferred embodiment of this invention, a printing plate comprising a photopolymerized stratum bonded by an adhesive stratum to a metal support, as described in Example V of U.S. Patent 3,036,913 and as illustrated in FIGURES 1 and 2 of this case, is treated t-o recover the metal support. This treatment comprises exposing the photopolymerizable stratum to actinic radiation followed by heating of the plate and then mechanically stripping the strata from the metal support. The amount of exposure to actinic, radiation can vary from about 4-Watt seconds/sq. in. to about 17.5-watt seconds/sq. in. The preferred amount of exposure will vary depending upon the thickness of the polymerizable stratum and the force required to strip the stratum from the plate. Greater exposure to actinic radiation further increases the operable maximum treating temperature and the time of heating. It also gives the stratum greater mechanical strength thereby yielding a cleaner separation `from the metal sup- ICC port. The temperature of heating the plate following the actinic irradiation can vary over a wide range. As the temperature becomes higher, the adhesive bond weakens so that less mechanical force is required to cause separation. The range of temperature will vary depending upon the thickness of the strata and the degree of exposure to actinic radiation.

This preferred embodiment is equally effective in recovering a metal support from partially fabricated printing plates, where only the adhesive stratum has been applied to the metal support. Defects are discovered at this stage of manufacture, in some instances, and it is, therefore, desirable to recover the metal support from such adhesive stratum without first applying the photopolymerizable stratum.

In another embodiment of this invention, a printing plate comprising a photopolymeriZab-le stratum bonded by an adhesive stratum to a metal support, as described in Example V of U.S. Patent 3,036,913 and as illustrated in FIGURES 1 and 2 of this case, is treated to recover the metal support. The treatment comprises heating the element to a temperature of from at least about F.

. to about 290 F. and preferably from about 170 F. to

about 250 F. for from about 20 seconds up to about 10 minutes and then stripping the coatings from the metal support. The heating can consist -of heating the lplate by soaking in hot water, by placing in an oven or by placing upon a hot plate.

This invention will be further illustrated but is not intended to be limited by the following examples, in addition to the examples appearing above, wherein the parts and percentages stated are by weight unless otherwise indicated.

EXAMPLE l A number of 6-inch by 6-inch photopolymeriz-able printing plates, each comprising a photopolymerizable stratum bonded by an adhesive stratum to a metal support, are prepared as described in Example V of U.S. Patent 3,012,952 except that the supporting sheets of aluminum are 103 mils in thickness.

The plates are fitted with copper-constantan thermocouples arranged to record the temperatures at the interface of the metal support and the adhesive stratum. The plates are placed on a conventional hot plate (metal support contacting the heating element), adjusted so that after about 10 minutes the thermocouples will register a fairly constant temperature of 160 F. One of the plates, lafter only 1 minute of heating, has an interface temperature of F. but the adhesive bond is not sufciently weakened `by the 1 minute treatment to permit mechanical separation of the adhering strata from the metal support without tearing said str-ata. Other plates are heated in this manner for varying times up to as much as l1 minutes but in all cases there is tearing of the adhering strata during the Iattempted mechanical removal of the strata. With a l6-minute treatment at the same adjustment of the hot plate, satisfactory stripping is o'btained. Other plates, with higher temperature adjustments of the hot plate heating element, are subjected to varying combinations of time and temperature of treatment as shown in Table I:

Table I Heating Time, Interface Tem- Mechanical Stripping Results Minutes perature, F.

Unsatisfactory, stratum tears. 160 Stratum easily removed in one piece. Do. to 1 290 Do.

3 EXAMPLE 2 Photopolymeriz-able printing plates prepared in Example 1 are exposed by a movable 1800-Watt high pressure mercury arc which passes over the plate 10 times 4 EXAMPLE 5 Photop-olymeriza-ble printing plates prepared as in Example 1 are recovered (without exposure to actinic radiation) 4by heating in hot water to soften the adhesive bond.

at a distance of 1% inches for a total exposure of 17.5- 5 Immediately after removal from the hot water ibath, the watt seconds/sq.in. of actinic radiation. lightly adhering strata are mechanically stripped away The plates are fitted with thermocouples to indicate to leave a reusable aluminum support. Treatment in the interface temperatures as in Example 1 and are water at 120 F. lfor 30 seconds causes a weakening of placed on a hot plate, preheated to 450 F., with the the adhesive bond but mechanical stripping is diicult. metal support contacting the heating element. The plates Satisfactory ystripping is achieved wihen the treatment is are allowed to remain on the hot plate for various lengths extended to 40 seconds. Very good results are obtained of time and said lengths of time, interface temperatures With other plates which are treated at rhifgher water temand results are recorded as shown in Table Il: peratures and for longer times including 45 seconds at Table Il 180 F., 6 minutes at 150 and 10 minutes at 160 F. As the treatment time and tempenature increase, the force necessary to cause mechanical stripping decreases. Length of Time on Interface Best results are obtained at temperatures between 130 Hot Plate Temlsetatufe' Mechanical Slflpplng Results F. and 190 E. with a treatment time of at least 1 minute. No beneficial results can be seen in extending the treat- 15 seconds. 250 l/[trips butfrequires gret force. 20 ment tune beyond about 20 manu/Les' ig sgilig.-

odrte orce require EXAMPLE 6 gg gg l striigsoiery easiiy. The procedure `disclosed in Example V of U.S. Patent 3,036,913 is repeated for a number of plates, `through the l steps of preparing the bonderized steel support and Another plate is heated 1n a similar manner but alaPpiying the `1limii thick stratum of ,adhesive (which lowed to remain on the hot plate for 5 minutes at which dries to a thicimess of 3 mils). No hhotopoiymerizahie time it has attained an interface temperature of 360 F. upper stratum is applied Fumes are first detected after 2 minutes heating when the rPhe plates ,are heated for V,mous times on .a hot 1.hate temperature Ihas reached 340 F. They continue to in- 30 with the metal Supp.Dit ,being in C(intact with tihe htiatiiig crease somewhat as heating is continued throughout the device and using a thermohoupie embedded `in the 1ath S-minute treatment period. At the end of the treatment hesiVe ayer to indicate the tsmptsratu,1re at the metah Period there is Practically no adhesion S0 the Polymer adhesive stratum interface at the end of the heating izable stratum is merely lifted ofr of the metal support. period. The ,plates are subsequently ybathed Lin trichiOi- Thus, from each of the printing plates described in this ethylene at room temperature Vfor 5 minutes with the ex. example, a clean support is recovered in satisfactory perimenta'l results being tabulated below:

Table III Time Temperature Solvent Stripping Results Control (no heating) 5 seconds 2 F. room temperature) 110 F Unsatisfactory, adhesive layer dissolves in solvent. Satisactory, nlm separates intact in the solvent.

condition for reuse in the manufacture of another printing plate.

EXAMPLE 3 Another printing plate as prepared in Example l is exposed to a line process negative and washed out as described in Example V of U.S. Patent 3,012,952. After being thoroughly dried, it is heated on a hot plate as described above for 1 minute to an intereface temperature of 300 F. The adherent str-ata are mechanically stripped olf intact to eifect satisfactory recovery of the metal support. In this case, the exposure through the line process negative causes suicient polymerization to give appreciable strengthening of the photopolymeriZa-ble stratum.

EXAMPLE 4 Several 18- by 24-inch photopolymerizable printing plates are prepared as in Example 1 of U.S. Patent 3,012,952, using supporting sheets of steel which are 12 mils in thickness. Three of these plates are immersed in hot Water (170 F.) for various lengths of time, viz., 2 minutes, 4 minutes and 5 minutes. All three can be mechanically stripped apart to yield the recovered metal support but, with the 2-minute treatment, considerable mechanical force is required. With the 4-minute treatment, stripping is much easier. With the S-minute treatment, stripping can be carried out with very little force. There is no advantage to be gained in extending the treatnient time beyond 5 minutes.

EXAMPLE 7 Although the adhesive stratum in the partially fabricated plates prepared in Example l6 contains only a thermal initiator and no photoinitiator, t-he monomer alone (triethyleneiglycol diacrylate) is suiiiciently ptiotopolymerizable that on exposure of 8 passes of the high pressure mercury arc, used as described in Example 2, :causes a reduction of unpolyrnerized monomer from the original 15.5% of the total composition to only 1.5% fby Weight. A plate, polymer-ized in this manner, is bathed in tiichloroethylene for 5 minutes at room temperature and the adhesive stratum separated in the solvent as an intact sheet, leaving a clean, reuseable metal support.

Tlhe triethyleneglycol diacrylate monomer in the original adhesive stratum was calculated from the weights of ingredients used in manufacture as being 15.5% by weight or" the total dry composition. After exposure, the remaining concentration of -unpolymerized monomer is determined analytically by Weiglhing a sample of the carefully dried adhesive stratum (2.4116 g), dissolving the soluble portion in glacial acetic aci-d, filtering and discarding the residue, and bringing up to Volume ml.) in a volumetric flask. One-fifth of lthe solution is transferred to a Z50-ml. volumetric flask to which is added 10 nil. of a 0.1 N solution of pyridine sulfate -dibromide (PSDB) and 5 ml. of mercurio acetate. The flask is kept in the dark for 30 minutes during which time the monomer is broniinated at the double bond by reaction with PSDB. Excess KI (3.0 g. 1dissolved in 85 ml. of H2O) is added to react with unreacted PSDB, forming KI3. The K13 is then titrated, in the presence of 4 ml. of an aqueous solution of stardh serving as an indicator, with an 0.1019 N solution of Na2S2O3 until a :colorless end point is reached. Since 8.0 ml. of the NaZS2O3 solution is required, the concentration of monomer is calculated as being 1.5% by weight of the dry sample of adhesive. This represents about the minimum degree of polymerization required to permit satisfactory recovery of the metal support, with the adhesive stratum separating as a single sheet. With less polymerization, the adlhesive sheet will dissolve in the solvent or come off in flakes so as to contaminate the solvent.

EXAMPLE 8 Table IV Water Teonperature, Soak Time,

Mechanical Stripping Results Minutes Borderline, strips in sections. Good, strips intact. Borderline, strips in sections. Goo), strips intact.

Do. Do.

EXAMPLE 9 A piece 'of nainsook is Iapplied over .the adhesive stratum of a partially fabricated printing plate prepared as described in Example 6. The adhesive is then thermally polymerized by heating in an oven for 5 minutes at 280 F. Next, the plate is soaked :in hot water for 3 minutes at 130 =F. The nainsook stripping aid, with the adhering adhesive layer, is then pulled away to leave a clean, reuseable steel support. Other plates are stripped satisfactorily in a simil-ar manner, using water temperatures up to 190 F. Less force is required in carrying out the stripping opera-tion as the time and temperature of the water treatment are increased, although no useful purpose appears to be served in extending the treatment time beyond 20 minutes. Comparable results are obtained when the nainsook strippin-g aid is replaced with a thin (0.001-inch thick) sheet of polyethylene terephthala-te illlm.

In the above examples, reference has been made to mechanical stripping of a stratum from a metal support. This may amount to no more than merely grasping by hand a corner of the stratum and then physically pulling the stratum free from the metal support. Prior to the stripping operation, the Iadhesive bond will have been weakened by heating while the Istratum will have been pollymerized to the extent that it :may be easily separated, as a single sheet, from the me-tal support. Various tools can be used to facilitate stripping, eg., pliers, clamps or other lgrasping tools, wedges, etc. A particularly useful device `is a key, .similar in principle to that used in opening a tin can. In using this device, `a corner or edge lof the stratum is inserted into a slot in the key which is then turned so as to wrap up the stratum as the tool is rolled along the plate surface, thereby effecting eparation 'of the stratum from the metal support.

T-he photopoly-merizable printing plates from which metal supports are reclaimed according to this invention can be the types described in U.S. Patents 2,923,673; 2,927,022 and 3,012,952. The adhesive stratum layers of these elements, which bond the photopolymerizable stratum .to the metal support, can be of the types described in Burg, U.S. Patent 3,036,913.

The metal supports reclaimed according to this invention are generally aluminum or steel but can be made of some other metals, such as magnesium and tin. The metal supports often have received special treatment to give them certain desired properties and Iuniform thickness. Such treatments increase the cost of the supports and th-us make their recovery Iof particular economic irnportance. Three types of metal support, speciali-ly treated to resist corrosion are described in Burg, U.S. Patent 3,036,913; an alodized laluminum sheet in Example Al, a bonderized steel sheet in Example 5, and a tin-plated steel sheet in Example 13.

The methods of this invention may be adapted to continuous processes for recovering metal supports, for example, by traversing through an loven or through solvent baths on a conveyor. Likewise, the mechanical stripping may be made a part of the continuous operation by manually separating the leading edges of the polymeriz- .able strata from the metall supports and -then feeding the leading edges of these strata and of the supports into different sets of pull rolls. Alternatively, separation might be started by bonding tabs to the leading edges of the strata, or the leading edges might be lifted by means of vacuum rolls prior to feeding the separated edges into separate sets of pull rolls.

Metal supports coated only with an unpoilymerized adhesive stratum can also be recovered by treatment with organic solvents at temperatures as low as room temperature, particularly when using chlorinated solvents such as methylene chloride, trichloroethylene, perchloroethylene and chloroform. Acetone and methyl ethyl ketone can also be used. Such recovery, however, is less eco nomical than the more preferred methods since it causes contamination of the solvent because of .the breaking up of the adhering stratum. Other recovery methods known in the art have proven entirely unsatisfactory, particularly in avoiding damage to the metal or in removing the adherin-g strata in a single sheet.

This invention is useful for the recovery and subsequent reuse -of a metal plate used for the support of polymerizable strata. I-t is particularly useful for the recovery and subsequent reuse of a metal plate used for the support of .a printing polymerizabile stratum.

An advantage of this invention is that it provides a practical and economical method for recovering a metall support in immediately reuseable condition `from a printing plate of the type containing po-lymerizable strata. A further beneiift is that such receovery of the metal support can be from either a partial or complete plate. Other advantages are simplicity and rapidity of operation.

While the methods herein described constitute preferred embodiments of the invention, it is to be understood that the invention is not limited to these precise methods, and that changes may be made therein without :departing fro-m the scope rof the invention which is 4defined in the appended claims.

What is claimed is:

1. A process for the recovery lof a metal support from the contiguo-us adhesive stratum of lan initial photoporlymerizable element havin-g an addition polymerizable stratum adherent to the adhesive stratum,

- (a) said polymerizable stratum comprising a solid photopoly-merizable llayer comprising a cellulose carboxylic acid ester, an ethylenically unsaturated monomer capable of forming a high polymer by photoinitiated addition polymerization 4and an addition polymerization 4initiator activatable Iby actinic light and thermally inactive -at C. and below, and

(b) said adhesive stratum consisting of (1) about 75 parts by weight of the copolyester .prepared by reacting an excess of ethylene ,glycol and dimethyl hexahydroterephthalate, tdi-methyl sebacate and dimethyl terephthalate, the Ilatter reactants in a molar ratio of 8:1:1 respectively, 2) about 2.4 parts by weight of benzoyl peroxide as a thermally sensitive 7 initiator, and (3) `about 22.5 parts by Weight of triethylene 'glycol ydiacrylate oontaining about 0.1 pant by Weight -of prnethoxyphenol as a thermal polymerization inhibitor,

( a) poiymerizin-g said adhesive stratum t-o the extent that the concentration 'of the triethylene glycol diacrylarte :in said `stratmrr is not more than 1.5% by weight on a dry basis, and

(fb) bathing said element in a chlorinated hydrocarbon solvent and separating the I'Ureafted adhesive Iin one piece by stripping it and any layers carried by lit `rom the supporting plate.

2. A process according to claim 1 wherein said solvent is trichiorroethy'lene.

References Cited by the Examiner UNITED STATES PATENTS 11/1955 Hedges et al 156-344 10/ 1961 Taylor et ai. 156--344 

1. A PROCESS FOR THE RECOVERY OF A METAL SUPPORT FROM THE CONTIGUOUS ADHESIVE STRATUM OF AN INITIAL PHOTOPOLYMERIZABLE ELEMENT HAVING AN ADDITION POLYMERIZABLE STRATUM ADHERENT TO THE ADHESIVE STRATUM, (A) SAID POLYMERIZABLE STRATUM COMPRISING A SOLID PHOTOPOLYMERIZABLE LAYER COMPRISING A CELLULOSE CARBOXYLIC ACID ESTER, AN ETHYLENICALLY UNSATURATED MONOMER CAPABLE OF FORMING A HIGH POLYMER BY PHOTOINITIATED ADDITION POLYMERIZATION AND AN ADDITION POLYMERIZATION INITIATOR ACTIVATABLE BY ACTINIC LIGHT AND THERMALLY INACTIVE AT 180*C. AND BELOW, AND (B) SAID ADHESIVE STRATUM CONSISTING OF (1) ABOUT 75 PARTS BY WEIGHT OF THE COPOLYESTER PREPARED BY REACTING AN EXCESS OF ETHYLENE GLYCOL AND DIMETHYL HEXAHYDROTEREPHTHALATE, DIMETHYL SEBACATE AND DIMETHYL TEREPHTHALATE, THE LATTER REACTANTS IN A MOLAR RATIO OF 8:1:1 RESPECTIVELY, (2) ABOUT 2.4 PARTS BY WEIGHT OF BENZOYL PEROXIDE AS A THERMALLY SENSITIVE INITIATOR, AND (3) ABOUT 22.5 PARTS BY WEIGHT OF TRIETHYLENE GLUCOL DICARYLATE CONTAINING ABOUT 0.1 PART BY WEIGHT OF P-METHOXYPHENYL AS A THERMAL POLYMERIZATION INHIBITOR, (A) POLYMERIZING SAID ADHESIVE STRATUM TO THE EXTENT THAT THE CONCENTRATION OF THE TRIETHYLENE GLYCOL DIACRYLATE IN SAID STRATUM IS NOT MORE THAN 1.5% BY WEIGHT ON A DRY BASIS, AND (B) BATHING SAID ELEMENT IN A CHLORINATED HYDROCARBON SOLVENT AND SEPARATING THE TREATED ADHESIVE IN ONE PIECE BY STRIPPING IT AND ANY LAYERS CARRIED BY IT FROM THE SUPPORTING PLATE. 